Due Date Calculator

How due date estimates are calculated

Most providers start with the first day of the last menstrual period (LMP) and add 280 days (40 weeks). Because not everyone ovulates on day 14, we also adjust the due date based on your typical cycle length. Longer cycles move the due date later, shorter cycles move it earlier. If you know the conception date, we add 266 days (38 weeks), which is the average time from fertilization to delivery.

Ultrasound dating can refine the estimate by measuring fetal size and comparing it to expected growth milestones. To use that method, input the ultrasound date and the gestational age reported on the scan. We then back-calculate the LMP and project forward 40 weeks.

Understanding gestational age

Gestational age counts from the first day of your last period, so it is about two weeks ahead of fetal age. For example, if conception occurred 10 weeks ago, gestational age is about 12 weeks. Tracking gestational age helps you follow trimester milestones, schedule prenatal screenings, and compare growth to clinical guidelines.

Tips for best accuracy

Use the most reliable date you have: an early ultrasound if cycles are irregular.
Update your cycle length to match your average over the last 3–6 cycles.
Remember that only about 5% of babies arrive on their exact due date.

Pair the due date estimate with BiteCheck to track nutrition, hydration, and pregnancy milestones week by week.

Frequently asked questions

How does the due date calculator work?

We use standard pregnancy dating rules. LMP adds 280 days and adjusts for cycle length, conception adds 266 days, and ultrasound back-dates from the gestational age at scan.

Why does the cycle length matter?

A 28-day cycle is the default. Longer cycles can shift ovulation later, so we add or subtract days to reflect your average cycle length.

Is ultrasound more accurate than LMP?

Early ultrasound dating is often more accurate than LMP if cycles are irregular or the LMP is unknown. Your clinician will choose the best estimate.

What is gestational age?

Gestational age counts from the first day of your last period, even though conception usually happens about two weeks later.

Can I use this for IVF or embryo transfer?

Yes. Use the conception date method and enter the transfer or fertilization date to estimate the due date.

Is this medical advice?

No. This calculator provides estimates only and should not replace medical guidance from your care team.

Informational only, not medical advice. Consult a healthcare professional for personalized guidance.